As above- check in: easy, security can be a mess, depending on the time of day/week, which convention just got out, etc.

Not sure if this tip still works, but you used to be able to use the security line for the C gates and then cut over to the tram for the D, IIRC.

http://www.mccarran.com/airportguide.asp

There was a three page thread over on the UA board about the D-gates:

Topic: Beware McCarran security to UA D-Gates in Las Vegas

"TSA is doing their very finest job to put UA out of business in Las Vegas with their third-world handling of security at the D gates at McCarran...."

http://www.flyertalk.com/forum/Forum50/HTML/017847.html